CS 1110: Introduction to Computing Using Python Lecture 18 Using Classes Effectively [Andersen, Gries, Lee, Marschner, Van Loan, White]
Announcements • A3 due tonight at 11:59pm. • Spring break next week: No office hours No consulting hours Limited piazza 3/30/17 Using Classes Effectively 2
Making Arguments Optional class Point3(object): • We can assign default values to """Instances are points in 3d space __init__ arguments x: x coord [float] Write as assignments to y: y coord [float] parameters in definition z: z coord [float] """ Parameters with default values are optional def __init__(self,x=0,y=0,z=0): • Examples : """Initializer: makes a new Point p = Point3() # (0,0,0) Precondition: x,y,z are numbers""" p = Point3(1,2,3) # (1,2,3) self.x = x p = Point3(1,2) # (1,2,0) self.y = y self.z = z p = Point3(y=3) # (0,3,0) … p = Point3(1,z=2) # (1,0,2) 3/30/17 Using Classes Effectively 3
Making Arguments Optional class Point3(object): • We can assign default values to """Instances are points in 3d space __init__ arguments x: x coord [float] Write as assignments to y: y coord [float] parameters in definition z: z coord [float] """ Parameters with default values are optional def __init__(self,x=0,y=0,z=0): • Examples : """Initializer: makes a new Point p = Point3() # (0,0,0) Precondition: x,y,z are numbers""" Assigns in order p = Point3(1,2,3) # (1,2,3) self.x = x p = Point3(1,2) # (1,2,0) self.y = y Use parameter name when out of order self.z = z p = Point3(y=3) # (0,3,0) … p = Point3(1,z=2) # (1,0,2) Can mix two approaches 3/30/17 Using Classes Effectively 4
Making Arguments Optional class Point3(object): • We can assign default values to """Instances are points in 3d space __init__ arguments x: x coord [float] Write as assignments to y: y coord [float] parameters in definition z: z coord [float] """ Parameters with default values are optional def __init__(self,x=0,y=0,z=0): • Examples : """Initializer: makes a new Point p = Point3() # (0,0,0) Precondition: x,y,z are numbers""" Assigns in order p = Point3(1,2,3) # (1,2,3) self.x = x p = Point3(1,2) # (1,2,0) self.y = y Use parameter name when out of order self.z = z p = Point3(y=3) # (0,3,0) … p = Point3(1,z=2) # (1,0,2) Can mix two approaches 3/30/17 Using Classes Effectively 5
On Tuesday, we learned how to make: • Class definitions • Class specifications • Class variables • Methods • Attributes (using self ) • A constructor with __init__ 3/30/17 Using Classes Effectively 6
Today • Class definitions • Class specifications • Class variables • Methods • Attributes (using self ) • A constructor with __init__ 3/30/17 Using Classes Effectively 7
Designing Types • Type : set of values and the operations on them int: ( set : integers; ops : +, –, *, /, …) Time ( set : times of day; ops : time span, before/after, …) Rectangle ( set : all axis-aligned rectangles in 2D; ops : contains, intersect, …) • To define a class, think of a real type you want to make 3/30/17 Using Classes Effectively 8
Making a Class into a Type 1. Think about what values you want in the set What are the attributes? What values can they have? 2. Think about what operations you want This often influences the previous question • To make (1) precise: write a class invariant Statement we promise to keep true after every method call • To make (2) precise: write method specifications Statement of what method does/what it expects (preconditions) • Write your code to make these statements true! 3/30/17 Using Classes Effectively 9
Planning out a Class: Time • What attributes ? • What invariants ? • What methods ? • What constructor ? (24-hour clock) 3/30/17 Using Classes Effectively 10
Planning out a Class class Time(object): Class Invariant """Instances represent times of day. Instance Attributes: States what attributes are present hour: hour of day [int in 0..23] and what values they can have. min: minute of hour [int in 0..59]""" A statement that will always be true of any Time instance. def __init__(self, hour, min): """The time hour:min. Pre: hour in 0..23; min in 0..59""" def increment(self, hours, mins): Method Specification """Move this time <hours> hours States what the method does. and <mins> minutes into the future. Gives preconditions stating what Pre: hours is int >= 0; mins in 0..59""" is assumed true of the arguments. def isPM(self): """Returns: this time is noon or later.""" 3/30/17 Using Classes Effectively 11
Implementing a Class • All that remains is to fill in the methods. (All?!) • When implementing methods: 1. Assume preconditions are true 2. Assume class invariant is true to start 3. Ensure method specification is fulfilled 4. Ensure class invariant is true when done • Later, when using the class: When calling methods, ensure preconditions are true If attributes are altered, ensure class invariant is true 3/30/17 Using Classes Effectively 14
Implementing an Initializer def __init__(self, hour, min): """The time hour:min. Pre: hour in 0..23; min in 0..59""" This is true to start You put code here Instance variables: This should be true hour: hour of day [int in 0..23] at the end min: minute of hour [int in 0..59] 3/30/17 Using Classes Effectively 15
Implementing an Initializer def __init__(self, hour, min): """The time hour:min. Pre: hour in 0..23; min in 0..59""" This is true to start A: B: C: Time.hour = hour hour = hour self.hour = hour Time.min = min min = min self.min = min Instance variables: D: hour: hour of day [int in 0..23] self.hour = Time.hour min: minute of hour [int in 0..59] self.min = Time.min This should be true at the end 3/30/17 Using Classes Effectively 16
Implementing a Method Instance variables: hour: hour of day [int in 0..23] This is true to start min: minute of hour [int in 0..59] What we are supposed def increment(self, hours, mins): to accomplish """Move this time <hours> hours and <mins> minutes into the future. This is also true to start Pre: hours [int] >= 0; mins in 0..59""" ? self.min = self.min + mins self.hour = self.hour + hours You put code here Instance variables: hour: hour of day [int in 0..23] This should be true min: minute of hour [int in 0..59] at the end 3/30/17 Using Classes Effectively 17
Implementing a Method Instance variables: hour: hour of day [int in 0..23] This is true to start min: minute of hour [int in 0..59] What we are supposed def increment(self, hours, mins): to accomplish """Move this time <hours> hours and <mins> minutes into the future. This is also true to start Pre: hours [int] >= 0; mins in 0..59""" self.min = self.min + mins You put code here self.hour = (self.hour + hours + self.min / 60) self.min = self.min % 60 self.hour = self.hour % 24 Instance variables: hour: hour of day [int in 0..23] This should be true min: minute of hour [int in 0..59] at the end 3/30/17 Using Classes Effectively 18
Example: class Time 3/30/17 Using Classes Effectively 19
Special Methods in Python class Point3(object): • __init__ for initializer """Instances are points in 3D space""" • __str__ for str() … • __repr__ for backquotes def __init__(self,x=0,y=0,z=0): • Start/end with 2 underscores """Initializer: makes new Point3""" … This is standard in Python Used in all special methods def __str__(self,q): Also for special attributes """Returns: string with contents""" … • For a complete list, see def __repr__(self,q): http://docs.python.org/refere """Returns: unambiguous string""" nce/datamodel.html … 3/30/17 Using Classes Effectively 20
Example: Converting Values to Strings str() Function Backquotes • Usage : str( <expression> ) • Usage : ` <expression> ` Evaluates the expression Evaluates the expression Converts it into a string Converts it into a string • How does it convert? • How does it convert? str(2) → '2' `2` → '2' str(True) → 'True' `True` → 'True' str('True') → 'True' `'True'` → "'True'" str(Point3()) → `Point3()` → '(0.0,0.0,0.0)' "<class 'Point3'> (0.0,0.0,0.0)" 3/30/17 Using Classes Effectively 21
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